I often get asked to help fellow photographers set up their cameras so they can take better pictures. Afterall, that is what we are all aspiring to do, right? Many things come to mind when first asked this vague question. Of course, they probably want a simple fix. There isn't one setting that fixes everything or that would obviously be a factory setting. However, the one setting that made the biggest difference in my photography was back button focus.
First, most DSLR’s, that I’ve ran across, have a button on the back of the camera that will focus the camera with autofocus if you push it. This is usually turned on from the factory. This is fine but the problem is that focus is usually also controlled by the shutter button. But having the shutter button click the picture and focus seems like a good plan until you don’t want it to do that.
For instance, maybe I want to do astrophotography and I get the stars perfectly focused manually then I push the shutter button to take the picture. Suddenly the camera tries to focus and it’s too dark so it 'hunts' for focus. You get a blurry picture and that’s frustrating. You could of course switch auto focus off but that’s a hassle and you might forget to switch it back on. Why have one more thing to think about? The fewer little things we have to remember, the more we can concentrate on creativity.
Another problem is is when you want to focus and recompose. There isn’t always a focal point exactly where you want it. If you try to do this without locking the focus the camera will try to refocus when you move the focal point off your subject. Again, we could focus, then switch autofocus off but then we have the same problem, or we are manually focusing and that slows down the process. In my opinion, that's less accurate. Especially when shooting with a narrow depth of field like 2.8 or faster. When using back button focus, the camera will only try to focus when you push that button. So, you put your focal point on your subject, press the back button to focus, then release the back button. Now, when you recompose your shot, your camera will not try to focus, it will simply capture the image.
A third issue is when the subject is moving closer or further from the lens. If you try to use the shutter button, it will be difficult to get the camera to focus on the subject in time to take the photo. This was frustrating to me when I started because I shoot rodeo type events where the contestants are always in motion. When we use the back button in conjunction with AI Servo (Canon), or similar with other manufacturers, we are telling the camera to keep the subject in focus the whole time. Then we push the shutter button whenever we feel the time is right.